Good question! The typical recommendation, and what I’ve been doing for years, is approximately a 1/4 cup of bleach for every 15 gallons of water your fresh water tank can hold.
For example, a 45-gallon freshwater tank would need about 3/4 cup (or about 6 ounces) of bleach for sanitization.
Here’s a quick how-to on sanitizing your freshwater system using bleach: https://camperfaqs.com/how-to-sanitize-rv-freshwater-tank-lines